This invention relates to the field of heavy duty radial pneumatic tires for trucks and truck trailers.
In many areas of the world, heavy duty vehicles such as trucks must have superior traction performance on the drive axle positions of the vehicle. Tires having sufficiently aggressive tread patterns with block element type tread patterns seem to provide the best traction performance.
As the tread becomes about half worn these block elements can lose some of their traction capability or performance. Ideally, the drive axle tires would be replaced at that time. Unfortunately, for the vehicle owner this means that the tread still had a useful half life remaining if it could be used in a less traction sensitive wheel position. Unfortunately, the original tread pattern when worn was not conducive to such applications such as trailer tires.
The need was apparent that tread patterns must change or convert when worn partially to a different pattern if they were to be successfully used in other wheel positions.
The use of convertible truck tires which start out having one block type tread pattern and as the tire reached a certain level of wear changed to a rib type tire was taught in U.S. Pat. No. 4,223,712 wherein less than full depth inclined transverse grooves would disappear after the tire was 30% to 70% worn.
A later U.S. Pat. No. 4,732,195 issued Mar. 22, 1988, taught that the transverse grooves should disappear toward the axially outside of the tread so that the separated blocks change into continuous ribs from the axially outside to the inside of the tread in sequence as the tread wears.
In U.S. Pat. No. 4,854,384 issued Aug. 8, 1989, taught a block type tread pattern which is also converted to a rib type tire when the tire is worn in the range of 35% to 75% of the maximum depth. The tread has a plurality of circumferentially continuous zig-zag and straight grooves which have a total width equal to 15-35% of the entire tread width when the tire is new and 10-25% of the entire tread width when the tire is worn and the transverse grooves completely disappear to form a rib pattern consisting solely of zig-zag longitudinal grooves.
In these patents the tread pattern simply was converted from block elements to ribs by the loss of the transverse groove.
These rib type tires generally will exhibit uniform wear and can be used where traction must be achieved by zig-zagging the longitudinal grooves. Historically, such tires have relatively poor braking traction when used on wet or snowy roads.
The present invention in a unique way converts a block element tread pattern for drive axles of trucks when the tire is half worn to a combination of shoulder ribs and central rows of larger block elements which can exhibit substantially superior wet traction capability than the prior art ribbed type tires.
A radial ply pneumatic tire 10 for trucks or trailers having a tread 12 with at least three longitudinal grooves 20,22 that extend circumferentially to divide the tread 12 into at least four parts is disclosed, more preferably 5 parts divided by four longitudinal grooves 20,22. The tread 12 has lateral grooves 30,31,32,33,34 extending across the at least four parts of the tread 12 to form at least four rows 1,2,3,4,5 of circumferentially separated blocks 40 including two shoulder rows 1,5 and at least two, preferably three central rows 2,3,4. Each block 40 has a radially outer surface 42.
The longitudinal grooves 20,22 have an average width W1 between the radially outer surface 42 of the tread blocks 40 extending to a depth of less than 60% of the total tread depth D thereafter the average width of the longitudinal grooves 20,22 narrows to less than 50% of W1 and the lateral grooves 30,34 in each shoulder row extends to a depth of less than 60% of the total tread depth.
The lateral grooves in the central rows 31,32,33 have an average length of L1 between the radially outer surface 42 of the tread blocks 40 to a depth of less than 60% of the total depth D, thereafter the lateral grooves 31,32,33 average length increases to greater than L1, preferably to L1+50% of W1 in length. The average groove width of the lateral grooves 31,32,33 in the central rows equals W3, the width W3 being measured between the radially outer surface 42 of the block 40 to a depth of less than 60% of the total depth D, thereafter the lateral grooves narrow to an average width W4 of less than 80% of W3 for the remainder of the groove depth, preferably less than 50% of W3.
These changes in average groove widths W1 of the longitudinal grooves 20,22 and the average groove widths W3 of the central row lateral grooves 31, 32, 33 to less than 50% W1 and less than 80% W3, respectively, preferably occurs at the same depth. Furthermore, it is preferred that the lateral grooves 30,34 in the shoulder rows 1,5 cease at the same depth wherein the longitudinal and the lateral grooves narrow.
It is further believed important that the longitudinal and lateral grooves change average width at 40% or more of the total depth D of the tread 12.
In the preferred embodiment tire each block element has at least one sipe 51,52,53 and a portion of the sipe 51,52,53 remains in the tread 12 after the width of the longitudinal 20,22 and lateral grooves 31,32,33 narrows due to the tire 10 being worn.
The tire 10 when new preferably has a net-to-gross ratio of less than 70% which increases by at least 10% at the depth of the tread where the average groove widths narrow, preferably increasing by almost 20%.
Definitions
xe2x80x9cAspect ratioxe2x80x9d of the tire means the ratio of its section height (SH) to its section width (SW) multiplied by 100% for expression as a percentage.
xe2x80x9cAsymmetric treadxe2x80x9d means a tread that has a tread pattern not symmetrical about the centerplane or equatorial plane EP of the tire.
xe2x80x9cCircumferentialxe2x80x9d means lines or directions extending along the perimeter of the surface of the annular tread perpendicular to the axial direction.
xe2x80x9cEquatorial plane (EP)xe2x80x9d means the plane perpendicular to the tire""s axis of rotation and passing through the center of its tread.
xe2x80x9cFootprintxe2x80x9d means the contact patch or area of contact of the tire tread with a flat surface at zero speed and under normal load and pressure.
xe2x80x9cGroovexe2x80x9d means an elongated void area in a tread that may extend circumferentially or laterally about the tread in a straight, curved, or zigzag manner. Circumferentially and laterally extending grooves sometimes have common portions. The xe2x80x9cgroove widthxe2x80x9d is equal to the tread surface area occupied by a groove or groove portion, the width of which is in question, divided by the length of such groove or groove portion; thus, the groove width is its average width over its length. Grooves may be of varying depths in a tire. The depth of a groove may vary around the circumference of the tread, or the depth of one groove may be constant but vary from the depth of another groove in the tire. If such narrow or wide grooves are of substantially reduced depth as compared to wide circumferential grooves which they interconnect, they are regarded as forming xe2x80x9ctie barsxe2x80x9d tending to maintain a rib-like character in the tread region involved.
xe2x80x9cInboard sidexe2x80x9d means the side of the tire nearest the vehicle when the tire is mounted on a wheel and the wheel is mounted on the vehicle.
xe2x80x9cLateralxe2x80x9d means an axial direction.
xe2x80x9cNet contact areaxe2x80x9d means the total area of ground contacting elements between defined boundary edges divided by the gross area between the boundary edges as measured around the entire circumference of the tread.
xe2x80x9cNet-to-gross ratioxe2x80x9d means the total area of ground contacting tread elements between the lateral edges around the entire circumference of the tread divided by the gross area of the entire tread between the lateral edges.
xe2x80x9cNon-directional treadxe2x80x9d means a tread that has no preferred direction of forward travel and is not required to be positioned on a vehicle in a specific wheel position or positions to ensure that the tread pattern is aligned with the preferred direction of travel. Conversely, a directional tread pattern has a preferred direction of travel requiring specific wheel positioning. xe2x80x9cAxialxe2x80x9d and xe2x80x9caxiallyxe2x80x9d means lines or directions that are parallel to the axis of rotation of the tire.
xe2x80x9cNon-Skid Depthxe2x80x9d means the full depth on total depth of the tread from the surface of the tread to the bottom of the deepest groove.
xe2x80x9cOutboard sidexe2x80x9d means the side of the tire farthest away from the vehicle when the tire is mounted on a wheel and the wheel is mounted on the vehicle.
xe2x80x9cRadialxe2x80x9d and xe2x80x9cradiallyxe2x80x9d means directions radially toward or away from the axis of rotation of the tire.
xe2x80x9cRibxe2x80x9d means a circumferentially extending strip of rubber on the tread which is defined by at least one circumferential groove and either a second such groove or a lateral edge, the strip being laterally undivided by full-depth grooves.
xe2x80x9cSipexe2x80x9d means small slots molded into the tread elements of the tire that subdivide the tread surface and improve traction.
xe2x80x9cTread elementxe2x80x9d or xe2x80x9ctraction elementxe2x80x9d means a rib or a block element.